Pelleting was introduced into the United States animal feed industry in the mid-1920's to improve feed utilization, increase the density of the feed and improve handling characteristics. Prior to about 1930 several different types of pelleting machines were utilized. Toward the end of the 1920's the so-called “flat die” pelleting machine and “ring die” pellet mill were developed in some of their early forms. While the flat die pelleting machine is still used for certain applications, the ring die pellet mill quickly became the preferred design and was rapidly adopted by the animal feed industry. It remains the type of pelleting machine of choice today. In addition to the ring die pellet mill itself, auxiliary equipment was developed including conditioners, cooler/dryers, and related process equipment.
Early pelleting processes involved mixing the feed ingredients into a mixture (a “mash”) and pelleting them with no further treatment. The rationale for this approach was to prevent modification or breakdown of vitamins and proteins due to the addition of heat to the feed mix. In the late 1930's some processors began subjecting animal feed mixes to water and steam by passing the mixtures through a conditioner prior to their introduction into the pellet extruders. The addition of steam during the conditioning improved production rates, reduced die wear, and improved pellet quality. Steam conditioning was quickly adopted by the industry and has remained an integral part of the pelleting process to the present time. In the conditioning step, live steam is injected into the mash as it is conveyed through the conditioner which generally consists of a cylindrical tube with a rotating shaft upon which numerous paddles or picks are mounted. The condensing steam increases the temperature and moisture content of the mash.
Adding fat to the diet of a ruminant may adversely affect the normal rumen fermentation process, and can prevent the normal breakdown of cellulosic material in the ruminant forestomach. However, adding fat to the ruminant diet can increase milk production.
Because fat is subject to melting and is not miscible with the water introduced as steam in the conditioning process, fat is generally considered to be harmful to the formation of a cohesive animal feed pellet. Accordingly, it is routine practice to limit the amount of fat included in feed pellets. In general, fat contents below 2 percent have little effect on the durability of the pellets. As the level increases above 2 percent, the pellets become increasingly softer, more mealy, and break apart more readily during handling. To manufacture pellets with a fat content above 2%, industry recommendations are to manufacture the pellets with up to 2% fat, and spray the remainder of the oil onto the finished pellets (Mommer et al., “A Guide to Feed Pelleting Technology,” Uniscope, Inc., Johnstown, Colo., USA, 2002).
Calcium salts of fatty acids were developed to overcome the problems associated with the decrease in rumen fermentation experienced when increasing levels of fats were added to ruminant rations. Calcium salts fatty acids are inherently less soluble in the rumen than conventional fats. Unfortunately, the calcium salts are slightly bitter and are considered less palatable than other fats when the calcium salts are fed in a loose or meal form. The calcium salts of fatty acids are granular products with a wide range of particle sizes, making it difficult to get them incorporated uniformly into a feed mix. Pelleting these products is generally believed to be impractical due to the high fat level in the products.